The present invention relates to an impregnated cathode used as a high current density cathode in an electron tube and to a process for producing the same.
An impregnated cathode used as a high current density cathode comprises a cathode base body made of a porous refractory metal such as tungsten (W), molybdenum (Mo) or the like impregnated with an electron emissive material such as a barium-calcium aluminate; a cup composed of a refractory metal such as tantalum (Ta), molybdenum or the like; and a cathode sleeve composed of a refractory metal such as tantalum, molybdenum or the like. The cathode base body is mounted on the cup, and the cup on which the cathode base body is mounted is inserted in the top portion of the cathode sleeve which is irradiated with a laser beam from the side thereof, such that the cathode sleeve, the cup and the cathode base body are welded together.
The cathode base body is composed of tungsten having a melting point of 3370.degree. C., and the cup and the cathode sleeve are composed of tantalum having a melting point of 2940.degree. C. or molybdenum having a melting point of 2617.degree. C. To weld them together, therefore, the welding portion must be heated at a temperature higher than the melting point of at least either one of the metals. However, the electron emissive material with which the cathode base body is impregnated has a melting point of about 1700.degree. C. During the welding, therefore, the electron emissive material melts and vaporizes, so that a hole is formed in the welding portion.
When the impregnated cathode was tested for its life being incorporated in an electron tube, the cutoff voltage changed greatly. The electron tube therefore was disassembled and investigated, and it was found that the cathode base body was split off from the cup and the sleeve even with a small force.
In order to cope with the above-mentioned problem, Japanese Patent Laid-Open No. 10823/1984 discloses a method in which a welding member is interposed between the cathode base body and the cup, and Japanese Patent Laid-Open No 111222/1984 discloses a method in which recessed portions are formed in the side walls of the cathode base body, and portions of the cup and the cathode sleeve corresponding to the recessed portions are irradiated with the laser beam, and protrusions of the cup and the cathode sleeve formed as they are melted are fitted to the recessed portions thereby to firmly hold the cathode base body.
These methods are to improve the existing method as it is difficult to directly weld together the cup, the cathode sleeve and the cathode base body composed of porous tungsten impregnated with the electron emissive material. Even with these methods, however, these members are not firmly adhered together.